1. Field of the Invention
This invention relates to an electronic marker for metallic valve box covers which provides a means by which the valve box cover can be located and uniquely identified.
2. Description of Prior Art
Buried valve boxes are commonly employed in gas and water distribution systems. There are a large number of valve boxes presently installed, and their number is increasing. To keep track of these installed valve boxes, some form of electronic location and identification is desirable. One common form of valve box consists of a metallic lid set in an all-metal riser. However, this type of valve box is problematic due to the substantial attenuation of radio waves caused by the metal.
A number of passive radio-frequency tagging systems are known. These systems consist of passive tags, or transponders, and an interrogating device. In operation, the interrogating device emits a pulse of RF energy of sufficient duration to xe2x80x9cchargexe2x80x9d any transponders in the immediate vicinity and then passively listens for any return signals. This cycle can be repeated numerous times. The interrogating device provides the operator with both visible and audible indications of proximity to the transponder. In cases where the transponder transmits an ID code, this code is also displayed by the interrogating device.
The majority of commercial tagging devices use a resonant antenna tuned to the same frequency as the interrogating device. Exploiting the effect of resonance allows the tag to absorb and store sufficient energy to produce a locating signal. The principle is that the resonant antenna will absorb more energy than it loses during each cycle of the radio wave generated by the interrogating device. In a properly constructed resonator, resistance losses and core losses will be low. Low losses means that the energy is available to perform location and identification functions.
Conventional electronic marker devices for buried valve boxes in use today comprise a tuned circuit sealed within a plastic envelope. The tuned circuit is made of a circularly wound coil of wire connected in parallel with a capacitor, with the assembly having a generally toroidal configuration. The coil is banded or tied at several peripheral locations. The plastic envelope has a generally xe2x80x9cUxe2x80x9d-shaped periphery with the side at the top of the xe2x80x9cUxe2x80x9d being initially open, and being sealed after the tuned circuit assembly has been inserted. The coil assembly may typically have a diameter of approximately twelve inches, with the xe2x80x9cUxe2x80x9d portion of the envelope interior being sized to receive the coil assembly peripheral shape with minimal distortion.
U.S. Pat. No. 4,761,656 teaches a passive marker device which is selectively placed relative to various portions of buried utilities such as gas, telephone, water and power lines, for use in locating such portions when necessary. The passive marker device includes a tuned circuit enclosed in a housing which includes a base member having an upstanding rim with a cover member bonded to the rim and to a central portion of the base member. An opening is provided that extends through the housing at the central portion of the housing. A boss which is recessed on the side of the base member that is away from the cover extends from the central portion of the base member to the cover member.
U.S. Pat. No. 6,049,279 teaches a conduit end cap adapted to house a transponder or electronic marker enabling the later location of a non-conductive plastic conduit end using above-ground sensors. The transponder conduit end cap includes a transponder housing having two flat, generally parallel walls defining a cavity, a transponder (either active or passive) located in the housing, and a fitting which attaches the transponder housing to the end of the conduit to thereby seal the conduit. The transponder axis is maintained by a predetermined orientation such as by partially filling the cavity with a liquid and using a buoyant envelope to contain the transponder.
U.S. Pat. No. 5,767,816 teaches a device which uses electromagnetic signals to mark or locate obscure articles, in particular, passive resonant circuits having ferrite cores which are used to mark buried cables and pipe. The passive electronic marker uses a plurality of ferrite cores to increase locating range and the cores are attached in such a manner that a slight bend of the marker at a junction between the cores does not significantly affect the marker""s performance. Stable locating range and frequency are achieved by controlling the air gap between the cores. By selecting the appropriate geometry for the core ends and winding interconnections, the first order permeability across the air gap is held constant during bending of the marker. See also U.S. Pat. Nos. 4,656,478, 4,334,227 and 5,099,227.
It is one object of this invention to provide an electronic marker for locating and identifying buried objects such as metallic valve boxes.
It is another object of this invention to provide a means for retrofitting a radio frequency location and identification tag to an existing all-metal valve box.
It is yet another object of this invention to provide an electronic marker for metallic valve boxes which addresses the problem of radio wave attenuation due to the shielding effect of the metal.
These and other objects of this invention are addressed in a valve box comprising a metallic riser and a metallic lid covering the top of the metallic riser by an electromagnetic resonator coupled to the valve box interior facing side of the metallic lid, whereby upon generation of an eddy current in the metallic lid, a current is generated in the electromagnetic resonator. The electromagnetic resonator preferably comprises an inductor wound on a ferrite rod in parallel with a capacitor. The key to operation of this invention is the degree of electromagnetic coupling between the electromagnetic resonator and the metallic lid.
The coupling is controlled by an air-gap between the ferrite rod and the bulk material of the lid. This coupling allows the transfer of energy between the lid and the resonator. The electromagnetic resonator stores energy while the interrogating device is active. The efficiency of the energy storage is proportional to the Q of the entire system. When the interrogating signal is removed, the stored energy in the resonator dissipates in the form of radio waves, as a result of which the location of the valve box can be identified.
The coupled system of the metallic lid and the electromagnetic resonator will always have a lower Q than the resonator alone. Consequently, the resonator must have a very high Q, where Q is a measure of the combined losses in the wiring and the core material. If the system is over-coupled, that is, the air-gap is too small, the Q of the system will approach that of the lid itself. The ohmic losses of the system will, thus, be high. The resonator will not store sufficient energy to radiate a location signal or power identification circuitry. If the system is under-coupled, due to a too large air-gap, insufficient energy will be transferred from the lid to the resonator. The Q of the system approaches the Q of the resonator, but the available energy is low.
The system of this invention can be modeled as a transformer with three windings. The primary winding that supplies the power is the interrogating coil. The lid can be modeled as a one-turn winding and part of the transformer core. The third winding is the coil of the resonator. The lid transfers energy from the interrogating coil to the resonator but has resistive losses. The resonator sees these losses as impedance to be matched to the impedance of the resonator. When impedance matching is achieved, the energy transferred to the resonator is maximized.